heap up vs stow away

heap up

verb
  • To make an untidy pile of (something). 

  • To increase over a period of time; to accumulate. 

stow away

verb
  • to stow or secure aloof in a safe place 

  • to obtain passage (on a ship or similar vehicle) by secreting oneself inside before departure 

How often have the words heap up and stow away occurred in a corpus of books? (source: Google Ngram Viewer )